Recently, plastic materials have been used for optical parts in place of inorganic glass, especially for eyeglass lenses, because they are lightweight and have good dyeability and good impact resistance. In particular, a polyethylene glycol bisallylcarbonate polymer (hereinafter referred to as CR-39) and polymethyl methacrylate (PMMA) have been used, primarily for plastic lenses. However, it has a refractive index of 1.50, which is lower than that of inorganic glass. Therefore, when they are used as lens materials, for example, the thickness of the lenses has to be increased with the increasing power. Consequently, not only is the superiority of plastics as lightweight material been impaired, but also they have not been preferred in view of the aesthetic property. Furthermore, the concave lens, in particular, has had problems when the thickness (edge thickness) of the surrounding portion of the lens is increased causing a birefringence or a chromatic aberration to occur.
For these reasons, in order to be able to decrease the thickness of the lens while applying most of the characteristics of plastics having a low specific gravity, a plastic material having a high refractive index has been in demand. As a material having such a performance, for example, (1) a polymer comprising a xylene diisocyanate compound and a polythiol compound (official gazette of JP-A-63-46213), (2) a resin comprising an aliphatic linear sulfur-containing diisocyanate and a polythiol compound (official gazette of JP-A-2-153302), (3) a polymer comprising a dithian derivative substituted with two isocyanatoalkyl groups and a polythiol (official gazette of JP-A-4-159275) are disclosed.
Although the resin in the above-mentioned item (1) and the resin in the above-mentioned item (2) have an increased refractive index by limiting a combination with a polythiol compound to be polymerized, the resin of item (1) has problems in that an Abbe number is decreased and a chromatic aberration is increased, and the resin of item (2) has a problem in that its heat resistance is decreased.
The polymer of item (3) has a high refractive index and a low dispersion (high Abbe number) and has an excellent heat resistance. However, in recent years, there has been a demand for a material having a higher refractive index and a lower dispersion.